Hope you are fine and in a good health.
I owned a blog www.ismag.com and today when I check my website in Google webmaster tool I found these errors in html improvement.
I am using WordPress SEO by Yoast. Please help me to solve this issue.
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Your case is not about the yoast plugin. You have duplicate on a lot of pages, with 2 slashes in the end. It seems the cause for this is your "share box" plugin or something like that, the one who gives the options for sharing/comments on the thumbnails, and more specifically - the comment one. Check for example http://www.ismag.com/category/technology/tech-news/, on the hover of any of the thumbnails, the comments lead to the wrong doubleslashed version. The fix should be easy, most probably there is slash in the plugin file referencing the comment button itself. I just have no idea which plugin is this, but anyway - this is your problem.